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Chapter 22. NNTP and the nntpd Daemon

Network News Transfer Protocol (NNTP) provides for a vastly different
approach to news exchange from C News and other news servers without native
NNTP support. Rather than rely on a batch technology like UUCP to transfer
news articles between machines, it allows articles to be exchanged via an
interactive network connection. NNTP is not a particular software package,
but an Internet standard described in RFC-977. It is based on a
stream-oriented connection, usually over TCP, between a client anywhere in
the network and a server on a host that keeps Netnews on disk storage. The
stream connection allows the client and server to interactively negotiate
article transfer with nearly no turnaround delay, thus keeping the number of
duplicate articles low. Together with the Internet’s high-transfer rates,
this adds up to a news transport that surpasses the original UUCP networks by
far. While some years ago it was not uncommon for an article to take two
weeks or more before it arrived in the last corner of Usenet; it is now often
less than two days. On the Internet itself, it is even within the range of
minutes.

Various commands allow clients to retrieve, send, and post articles. The
difference between sending and posting is that the latter may involve articles
with incomplete header information; it generally means that the user has just
written the article.[132]
Article retrieval may be used by news transfer clients as well as newsreaders.
This makes NNTP an excellent tool for providing news access to many clients on
a local network without going through the contortions that are necessary when
using NFS.

NNTP also provides for an active and a passive way to transfer news,
colloquially called “pushing” and “pulling.” Pushing
is basically the same as the ihave/sendme protocol used by C News (described in
Chapter 21). The client offers an article to the server
through the IHAVE msgid
command, and the server returns a response code that indicates whether
it already has the article or if it wants it. If the server wants the
article, the client sends the article, terminated by a single dot on a
separate line.

Pushing news has the single disadvantage that it places a heavy load on
the server system, since the system has to search its history database for
every single article.

The opposite technique is pulling news, in which the client requests a
list of all (available) articles from a group that have arrived after a
specified date. This query is performed by the
NEWNEWS command. From the returned
list of message IDs, the client selects those articles it does not yet have,
using the ARTICLE command for
each of them in turn.

Pulling news needs tight control by the server over which groups and
distributions it allows a client to request. For example, it has to make
sure that no confidential material from newsgroups local to the site is sent
to unauthorized clients.

There are also a number of convenience commands for newsreaders that
permit them to retrieve the article header and body separately, or even
single header lines from a range of articles. This lets you keep all
news on a central host, with all users on the (presumably local) network
using NNTP-based client programs for reading and posting. This is an
alternative to exporting the news directories via NFS, as described
in Chapter 21.

An overall problem of NNTP is that it allows a knowledgeable person to insert
articles into the news stream with false sender specification. This is
called news faking or
spoofing.[133]
An extension to NNTP allows you to require user authentication for
certain commands, providing some measure of protection against people abusing
your news server in this way.

There are a number of NNTP packages. One of the more widely known is
the NNTP daemon, also known as the
reference implementation.
Originally, it was written by Stan Barber and Phil Lapsley to illustrate the
details of RFC-977. As with much of the good software available today, you may
find it prepackaged for your Linux distribution, or you can obtain the source
and compile it yourself. If you choose to compile it yourself, you will need to
be quite familiar with your distribution to ensure you configure all of the
file paths correctly.

The nntpd package has a server, two clients for
pulling and pushing news, and an inews
replacement. They live in a B News environment, but with a little
tweaking, they will be happy with C News, too. However, if you plan to
use NNTP for more than offering newsreaders access to your news server,
the reference implementation is not really an option. We will therefore
discuss only the NNTP daemon contained in the nntpd package
and leave out the client programs.

If you wish to run a large news site, you should look at
the InterNet News package, or INN, that was written by
Rich Salz. It provides both NNTP and UUCP-based news transport. News transport is definitely better than
nntpd. We discuss INN in detail in
Chapter 23.

The NNTP Protocol

We’ve mentioned two NNTP commands that are key to how news articles are pushed
or pulled between servers. Now we’ll look at these in the context of an actual
NNTP session to show you how simple the protocol is. For the purposes of our
illustration, we’ll use a simple telnet client to connect to
an INN-based news server at the Virtual Brewery called
news.vbrew.com. The server is running
a minimal configuration to keep the examples short. We’ll look at how to
complete the configuration of this server in Chapter 23.
In our testing we’ll be very careful to generate articles in the
junk newsgroup only, to avoid
disturbing anyone else.

Connecting to the News Server

Connecting to the news server is a simple as opening a TCP connection to
its NNTP port. When you are connected, you will be greeted with a welcome
banner. One of the first commands you might try is help. The response you get generally depends upon whether
the server believes you are a remote NNTP server or a newsreader, as there are
different command sets required. You can change your operating mode using the
mode command; we’ll look at that in a
moment:

The responses to NNTP commands always end with a period (.) on a
line by itself. The numbers you see in the output listing are
response codes and are used by the server to indicate success or
failure of a command. The response codes are described in RFC-977; we’ll talk
about the most important ones as we proceed.

Pushing a News Article onto a Server

We mentioned the IHAVE command
when we talked about pushing news articles onto a news server. Let’s
now have a look at how the IHAVE command actually works:

ihave <123456@gw.vk2ktj.ampr.org>
335
From: terry@gw.vk2ktj.ampr.orgSubject: test message sent with ihaveNewsgroups: junkDistribution: worldPath: gw.vk2ktj.ampr.orgDate: 26 April 1999Message-ID: <123456@gw.vk2ktj.ampr.org>Body: This is a test message sent using the NNTP IHAVE command..
235

All NNTP commands are case insensitive, so you may enter them in
either upper- or lowercase. The IHAVE command takes one
mandatory argument, it being the Message ID of the article that is being pushed. Every news article is assigned a unique message ID when
it is created. The IHAVE command provides a way of the NNTP server to say which articles it has
when it wants to push articles to another server. The sending server
will issue an IHAVE command
for each article it wishes to push. If the command response code
generated by the receiving NNTP server is in the “3xx”
range, the sending NNTP server will transmit the complete article,
including it’s full header, terminating the article with a period on a
line by itself. If the response code was in the “4xx”
range, the receiving server has chosen not to accept this article,
possibly because it already has it, or because of some problem, such
as running out of disk space.

When the article has been transmitted, the receiving serve issues another
response code indicating whether the article transmission was successful.

Changing to NNRP Reader Mode

Newsreaders use their own set of commands when talking to a
news server. To activate these commands, the news server has to be
operating in reader mode. Most news servers default
to reader mode, unless the IP address of the connecting host is listed
as a news-forwarding peer. In any case, NNTP provides a command to
explicitly switch into reader mode:

NNTP reader mode has a lot of commands. Many of these are designed to
make the life of a newsreader easier. We mentioned earlier that there are
commands that instruct the server to send the head and the body of articles
separately. There are also commands that list the available groups and
articles, and others that allow posting, an alternate means of sending news
articles to the server.

Listing Available Groups

The list command lists a number
of different types of information; notably the groups supported by the server:

list newsgroups
215 Descriptions in form "group description".
control News server internal group
junk News server internal group
local.general General local stuff
local.test Local test group
.

Listing Active Groups

list active shows each supported group and provides information
about them. The two numbers in each line of the output are the high-water
mark and the low-water mark—that is, the highest numbered article and
lowest numbered article in each group. The newsreader is able to form an
idea of the number of articles in the group from these. We’ll talk a little
more about these numbers in a moment. The last field in the output displays flags
that control whether posting is allowed to the group, whether the group is
moderated, and whether articles posted are actually stored or
just passed on. These flags are described in detail in Chapter 23.
An example looks like this:

Posting an Article

We mentioned there was a difference between pushing an article and posting
an article. When you are pushing an article, there is an implicit assumption
that the article already exists, that it has a message identifier that has
been uniquely assigned to it by the server to which it was originally posted,
and that it has a complete set of headers. When posting an article, you are
creating the article for the first time and the only headers you supply are
those that are meaningful to you, such as the Subject and the Newgroups to
which you are posting the article. The news server you post the article on
will add all the other headers for you and create a message ID that it will
use when pushing the article onto other servers.

All of this means that posting an article is even easier than pushing one.
An example posting looks like this:

We’ve generated two more messages like this one to give our following examples
some realism.

Listing New Articles

When a newsreader first connects to a new server and the user chooses a
newsgroup to browse, the newsreader will want to retrieve a list of new
articles, those posted or received since the last login by the user. The
newnews command is used for this
purpose. Three mandatory arguments must be supplied: the name of
the group or groups to query, the start date, and the start time from which
to list. The date and time are each specified as six-digit numbers, with the
most significant information first;
yymmdd and
hhmmss, respectively:

Selecting a Group on Which to Operate

When the user selects a newsgroup to browse, the newsreader may tell the
news server that the group was selected. This simplifies the interaction
between newsreader and news server; it removes the need to constantly
send the name of the newsgroup with each command. The
group command simply takes the name of the selected group as
an argument. Many following commands use the group selected as the
default, unless another newsgroup is specified explicitly:

group junk
211 3 1 3 junk

The group command returns a message
indicating the number of active messages, the low-water mark, the high-water
mark, and the name of the group, respectively. Note that while the number of
active messages and the high-water mark are the same in our example, this is
not often the case; in an active news server, some articles may have expired
or been deleted, lowering the number of active messages but leaving the
high-water mark untouched.

Listing Articles in a Group

To address newsgroup articles, the newsreader must know
which article numbers represent active articles. The
listgroup command offers a list of the active
article numbers in the current group, or an explicit group if the group name is
supplied:

listgroup junk
211 Article list follows
1
2
3
.

Retrieving an Article Header Only

The user must have some information about an article before she can know
whether she wishes to read it. We mentioned earlier that some commands allow the article header and body to be transferred separately. The
head command is used to request that the server transmit
just the header of the specified article to the newsreader. If the user
doesn’t want to read this article, we haven’t wasted time and network
bandwidth transferring a potentially large article body unnecessarily.

Articles may be referenced using either their number (from the
listgroup command) or their message identifier:

Retrieving an Article Body Only

If, on the other hand, the user decides she does want to read the article, her
newsreader needs a way of requesting that the message body be transmitted. The
body command is used for this purpose. It operates in much
the same way as the head command, except that only the message
body is returned:

Reading an Article from a Group

While it is normally most efficient to separately transfer the headers
and bodies of selected articles, there are occasions when we are better off
transferring the complete article. A good example of this is in applications
through which we want to transfer all of the artices in a group without any sort of
preselection, such as when we are using an NNTP cache program like
leafnode.[134]

Naturally, NNTP provides a means of doing this, and not surprisingly, it
operates almost identically to the head command as well.
The article command also accepts an article number or
message ID as an argument, but returns the whole article including its header:

If you attempt to retrieve an unknown article, the server will return
a message with an appropriately coded response code and perhaps a readable
text message:

article 4
423 Bad article number

We’ve described how the most important NNTP commands are used in this section.
If you’re interested in developing software that implements the NNTP protocol,
you should refer to the relevant RFC documents; they provide a great deal of
detail that we couldn’t include here.

Let’s now look at NNTP in action through the nntpd server.

[132] When posting an
article over NNTP, the server always adds at least one header field, NNTP-Posting-Host:. The field contains the client’s hostname.

[133] The same problem exists with the Simple Mail Transfer Protocol (SMTP), although
most mail transport agents now provide mechanisms to prevent spoofing.